The present invention relates generally to the hardening of polymer thin films and more specifically to the hardening of patterned photoresist used in microelectronics, flat panel displays, and optoelectronics processing. Both latent images, as well as resist relief patterns, formed in an earlier lithographic process need to be protected from subsequent high temperature processing which causes reflow of resist and results in loss of pattern fidelity. This high temperature processing occurs intentionally as a consequence of a subsequent process step. During this subsequent process step, depending on the chemistry of the resist, and the temperature-time cycle to which it is subjected, the resist relief pattern itself or the latent images contained therein flow, altering or destroying the pattern integrity of the first layer. Examples of this subsequent process step include hard baking of patterned resist or resists with latent images, dry etching or reactive ion etching, high temperature sputtering, ion implantation, and lift-off process steps. Each of these process steps has a unique temperature-time cycle associated with it and may cause undesired reflow of the original underlying resist.
Uniformity both across the planar area of the resist film and versus depth into the resist film are lacking in prior art resist hardening methods. Ultraviolet (UV) photohardening of resists has been proposed in the prior art where the chemistry of the resist is changed by absorption in the resist of UV or vacuum ultraviolet (VUV) radiation. In the UV photohardening prior art, (H. Hiraoka and J. Pacansky, J. Electrochemical Soc., 128, 2645, 1981) it is known to use 250 nm photons to thermophysically harden AZ-type photoresists through a photo-oxidation hardening mechanism. UV photohardening, while practical for treating thin resist layers is not suitable for hardening thick layers in a uniform fashion across the planar area of the resist, as described above, and especially versus depth into the resist. The optical extinction coefficient of most organic resist constituents is around 1.0.times.10.sup.4 liter/molcm, and 95% of the ultraviolet energy from the lamp is absorbed within 3000 Angstroms of the surface. Hence, for thick resists non-uniform exposure occurs versus depth when using photons to harden the resist. The exposure time and deposited energy required to harden using photons are also considerable. Finally, to produce the required UV energy at a wavelength around 250-300 nm., from a conventional UV lamp, the input electrical energy needed is large since the UV energy/electrical power conversion ratio in conventional UV lamps is typically 10.sup.-3. Hence, a 1-watt photon output may require kilowatts of input electrical power. The typical operating life of these lamps is limited to around 1000 hours, and they require external cooling during their operation.
Plasma hardening schemes such as PRIST (Plasma Resist Image Stabilization Technique, W.H.-L. Ma, U.S. Pat. No. 3,920,483) have been proposed in the prior art. In these techniques the chemistry of the resist is modified by bombardment of the resist by reactive and charged species produced in the plasma. Plasma hardening of resist is also limited to shallow depths. A 1-kilovolt electron can, for example, penetrate a typical resist to a depth of less than 0.03 um, and an ion with a similar energy can penetrate the resist even less. However, electrons and ions produced in a typical plasma possess energies typically less than 100 volts and hence are ineffectual in hardening uniformly any but the thinnest top layer. Further, there is the problem of distortion or shift of the geometrical pattern during the plasma hardening process, which should be minimized.
Ion beam hardening, still in the research stage, is also depth limited even at 100 kV ion energy due to the shallow ion penetration depth into polymer films. This depth is about 1 micrometer, depending on the type and energy of the ions, as illustrated in the table below, which shows illustrative data for gallium (heavy) and hydrogen (light) ions. See H. Ryssel, K. Haberger, and H. Krantz, J. Vac. Sci. Technology 19(4)m 1358 (1981); R. L. Selliger and H. Krantz, Electronics p. 142, (1980); and 1. Adesida, C. Anderson, and E. D. Wolf, J. Vac. Sci. Technology 1, 1182 (1983). Ions, if they do penetrate, can damage the underlying substrate due to their large mass.
TABLE 1 ______________________________________ ION PENETRATION ION ENERGY (kV) DEPTH (um) ______________________________________ Ga 40 0.046 55 0.060 120 0.120 + H 40 0.52 100 1.08 120 1.12 ______________________________________
In summary, the problems with flood exposure of photoresist with ion beams include radiation damage of the underlying substrate for thin resist films, non-uniformity of exposure over wide areas, depth non-uniformity, excessive resist heating and/or excessive resist crosslinking making resist stripping more difficult. All of the above lead to loss of process reliability.
High energy electron beam resist hardening systems have the advantage of deeper penetrating power of the energetic electrons, compared to ions, into the resist, for a given accelerating voltage, allowing for more uniform exposure versus depth. Empirically, the depth of electron penetration, R (in microns), into the polymer film is related to the density of the polymer and the incident electron energy through the equation R=0.046V.sup.1.7 /d, where V is the energy of the beam in kilovolts, and d is the density of the resist in gms/cc. See A. Novembre and T. N. Bowmer, Materials for Microlithography, Chap. 11, p. 241; and C. G. Wilson, L. F. Thomson and J. M. J. Frechet, ACS Symposium Series, 266, Washington, C.C. (1984). Note that polymer resists with a density around 1.2 grams/cc. can be penetrated to a depth of 11 um. by 25 kilovolt electrons, allowing for very uniform depthwise polymer exposure over at least 5 microns. A second major advantage of electron beam hardening is the ability to produce very spatially uniform electron beams over very wide areas, as described below for non-thermionic electron sources. Despite the basic advantages listed above, a conventional electron beam hardening system has several practical disadvantages, as presently practiced, that have restricted its use to date.
One disadvantage of prior art electron beam hardening systems is that they use a hot filament source for electron emission and therefore require operation in high vacuum (10.sup.-6 to 10.sup.-4 Torr conditions) or the filament will be destroyed. The photoresist electron beam hardening process, especially when practiced over wide areas, produces vapors which will poison the thermionic electron emission process. In addition to catastrophic poisoning, the vapors in small quantities substantially reduce the operating life of the filament to unacceptable levels. Another disadvantage of thermionic electron beam systems is that the electron dose needed to harden a typical resist of 3 um. thickness could be as much as 600 to 1000 uc/cm..sup.2, which is equivalent to a one minute exposure of a beam with a current density of more than 1 mA/cm..sup.2 A beam with lower current density would take a correspondingly longer processing time. For hardening large surface areas, a large area electron beam would be needed and keeping filament temperature constant over a large area is by no means easy. Finally, thermionic electron sources have difficulty in providing wide area electron beams with intensity uniformity less than plus or minus 5%. As a consequence, beam uniformity and beam brightness become problematic for hot filament wide area electron sources.
In summary, the prior art resist or polymer hardening processes are limited to hardening thin (0.1 to 1 micron) layers of the polymer or resist which is clearly insufficient for the thicknesses envisaged in many technological applications including packaging lithography. Furthermore, uniformity of hardening both across the wafer and through the depth of the patterned resist are unresolved issues in prior art methods.